1. Field of the Invention
This invention relates to a lamp string for Christmas, and particularly to a backstop socket structure to prevent a lamp string from turning-off upon a bulb dropped unintentionally.
2. Description of the Prior Art
The conventional Christmas lamp string is usually made of an elongate lamp string, which includes a plurality of separate lamp strings connected together; each separate lamp string includes a plurality of short power-supply wires connected between two sockets. The first bulb of the lamp string is connected, by using a longer wire, with the plug; the longer wire is twisted with the lamp string to form into a separate lamp string.
Each of the sockets in the lamp string is to be plugged with a connector, which is mounted with a bulb so as to facilitate the bulb to be replaced in case of being burned out. The number of sockets and the coefficient of resistance of each bulb are all pre-designed in accordance with the voltage and current of a given area.
The plug of each lamp string is furnished with a fuse to prevent the power-supply wire of the lamp string from being over-loaded. In case of the power-supply wire having a short circuit or being over-loaded, the fuse in the plug will be burned out automatically so as to avoid a hazard; however, the fuse furnished in the plug is not designed to prevent the socket from being overloaded.
The bulb plugged in the socket of each lamp string has two copper wires to be fixed in place with a positioning bead; the tail ends of the copper wires are mounted with a tungsten filament; the aforesaid parts are then mounted in a glass tube, of which both ends are to be sealed by means of different welding methods respectively so as to form into a bulb; the bulb is to be plugged in the socket of the lamp string. Since the sockets of the lamp string are connected one another in series, the whole lamp string is subject to having an open circuit and outage in the event of a tungsten filament being burned out.
In order to avoid the lamp string to turn off upon the tungsten filament of a bulb being burned out, an aluminum fuse of 0.065 m/m is wound around the two copper wires near the positioning bead; the number of turns of the fuse is designed in accordance with technical requirement, but it has at least 2.5 turns to enable the fuse to mount in place. The object of furnishing such a fuse is to maintain the whole lamp string to be in lighting-up condition in case of the tungsten filament being burned out; in that case, the fuse having lower resistance can still have the two copper wires maintained in conduction condition. The requirement of at least 2.5 turns of the aluminum fuse is to prevent the fuse from being burned out upon the current being not over a given value.
The aluminum fuse mounted between the two copper wires and near the positioning bead must have a resistance less than that of the tungsten; in case of the tungsten filament being burned out, the aluminum fuse can still maintain a current to flow through the two copper wires so as to avoid the lamp string to have an outage for a short time; however, since every bulb in the lamp string will lose at least a portion of the tungsten filament to share the power dissipation, the tungsten filaments of the rest bulbs will have a higher power dissipation; in other words, the serviceable life of the tungsten filament in the bulb will be reduced proportionally. Whenever the number of bulbs in a lamp string is reduced gradually, the bulb number of bulbs, which are not lit up, will be increased. Since the power dissipation of every bulb is increased, the temperature thereof will also be increased; then, the temperature of the connector of each bulb will be increased to an over-loaded condition. Generally, the material used for making the connector and the socket will be improved to withstand a given high temperature; in that case, the cost for the material thereof will be increased without solving the problem of a single bulb in a lamp string to suffer from a high temperature.
In a conventional lamp string, if one bulb is dropped or missing, the whole lamp string will be turned off immediately.